FOSSILS OF THE COAL MEASURES. 585 



].i><-nlity and position Roof of Coals Xos. 4 and 5 of the Illinois sec- 

 tion, in Fulton county, and in the shafts at Springfield. 



Genus ALLORISMA, King. 

 ALLORISMA COSTATA, M. and W. 



PL 26, Fig. 15. 

 Alloritmi costata. MEEK and WORTHED. Proceed. Acad. Nat. Soi., Phila., 1669, p. 171. 



SHELL under medium size, longitudinally oblong, the 

 length being more than twice the hight, very thin, rather 

 convex in the central uinbonal regions; anterior margin 

 rather short, closed, and narrowly rounded; basal margin 

 forming a long, nearly semi-elliptic curve, with a very slight 

 sinuosity in front of the middle; posterior side .compressed, 

 but apparently a little gaping and distinctly truncated 

 nearly vertically from the base about half way up, and 

 thence a little obliquely forward and upward to the dorsal 

 margin; posterior dorsal region compressed above the um- 

 bonal ridge; cardinal margin equaling about two -thirds the 

 entire lengjh of the shell, very nearly straight, and inflected 

 so as to form a narrow or lance-linear corselet, extending 



o 



its whole length; beaks convex, rising a little above the 

 cardinal margin, and placed slightly more than one-sixth 

 the length of the valves behind the anterior extremity; 

 hmule well defined and lance-ovate in form. Surface orna- 

 mented by about twenty-five very regularly arranged, dis^ 

 tiuctly elevated, concentric cost*, which commence near 

 the lunule and extend backward parallel to the base, to 

 the well defined, angular uinbonal ridge leading from the 

 beaks to the posterior basal extremity, at which ridge they 

 suddenly obsolete, or very nearly so, being mainlv 

 on the more compressed posterior dorsal region 

 by distinct lines of growth, which are crossed on the middle 

 of this area by a second oblique linear ridge extending from 

 the beaks to the middle of the posterior margin. Some 



